The present invention relates to a process for preparing a polyvinyl alcohol having a high degree of polymerization, and more particularly to a process for preparing a substantially particulate polyvinyl alcohol having a high degree of polymerization which comprises hydrolyzing a substantially particulate polyvinyl ester having a high degree of polymerization in a heterogeneous system.
A polyvinyl alcohol is obtained, as is known, by hydrolyzing a polyvinyl ester and it is required to hydrolyze a polyvinyl ester having high degree of polymerization in order to obtain a polyvinyl alcohol having a high degree of polymerization.
As methods for obtaining a polyvinyl ester having a high degree of polymerization, there are known a bulk polymerization of a vinyl ester at low temperature [as described in, for instance, Sakusanbinirujushi (Vinyl acetate resin), 69-78, edited by Ichiro Sakurada], a suspension polymerization of a vinyl ester at low temperature (as described in, for instance, Sakusanbinirujushi, 85-87 or Japanese Unexamined Patent Publication No. 148209/1986), a photopolymerization of a vinyl ester at low temperature (as described in, for instance, Japanese Examined Patent Publication No. 17848/1960, U.S. Pat. No. 4,463,138 or Japanese Unexamined Patent Publication No. 64807/1987), and the like.
In order to obtain the polyvinyl alcohol having a high degree of polymerization from the polyvinyl ester obtained according to each above-mentioned methods, the polyvinyl ester is dissolved in an alcohol and then hydrolyzed with an alkali catalyst in a usual manner. However, when conducting the methods on an industial scale, it is required to limit the concentration of the alcohol solution of polyvinyl ester to a very low concentration, such as from 5 to 6% by weight at the highest as described in, for instance, Japanese Unexamined Patent Publication No. 148209/1986 or No. 64807/1987. This is so because, it is difficult to stir the alcohol solution of the polyvinyl ester which does not have a low concentration of the polyvinyl ester in the alcohol due to its very high viscosity. Accordingly, it is required to recover a large amount of a solvent, which is when operating the method industrially.
On the other hand, as methods for preparing a particulate polyvinyl alcohol, there have hitherto been known a method in which polyvinyl ester beads are dispersed in a mixture of methanol containing an alkali catalyst and a liquid with which methanol is partially compatible, in which a polyvinyl alcohol cannot be dissolved, and in which a polyvinyl ester can be slightly dissolved, such as liquid petroleum hydrocarbons, thereby hydrolyzing the polyvinyl ester, as disclosed in, for instance, U.S. Pat. No. 2,502,715; a method in which a paraffinic hydrocarbon having a boiling temperature of not less than 30.degree. C. is added to an alcohol solution of a polyvinyl ester to emulsify or disperse the alcohol solution in the paraffinic hydrocarbon and the hydrolysis of the polyvinyl ester is conducted by using an alkali catalyst as disclosed in BP 749,458; and the like.
The above-mentioned methods, however, have the drawback that it is required to remove the liquids such as liquid aliphatic, aromatic or naphthene hydrocarbons, which are used in the hydrolysis step, from the reaction mixture by a complex removing step.
An object of the present invention is to provide a process for industrially and easily preparing a particulate polyvinyl alcohol having a high degree of polymerization, which is high in a bulk density and excellent in a powder flowability, by efficiently hydrolyzing a polyvinyl ester having a high degree of polymerization.
This and the other objects of the present invention will become apparent from the description hereinafter.